Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker is comprised of two major components. One component is the device itself which includes pulse generator circuitry that generates the pacing stimulation pulses, other circuitry that senses cardiac activity, and a power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing polarity electrode configurations. In unipolar pacing, the pacing stimulation pulses are applied between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. Usually the electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In bipolar pacing, the pacing stimulation pulses are applied between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, one electrode serving as the anode and the other electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events represented as P waves on the surface electrocardiogram (ECG) and intrinsic ventricular events represented as R waves on the surface ECG. The pacemaker, however, does not use the surface ECG electrical events but uses the signal as identified inside the heart. This is termed an electrogram. It would be an atrial EGM (AEGM) for the native atrial depolarization and a ventricular EGM (VEGM) for a native ventricular depolarization. By monitoring such AEGM and VEGM, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
The energies of the applied pacing pulses are selected to be above the pacing energy stimulation threshold of the respective heart chamber to cause the heart muscle of that chamber to depolarize or contract. If an applied pacing pulse has an energy below the pacing energy stimulation threshold of the respective chamber, the pacing pulse will be ineffective in causing the heart muscle of the respective chamber to depolarize or contract. As a result, there will be failure in sustaining the pumping action of the heart. It is therefore necessary to utilize applied pacing pulse energies which are assured of being above the pacing energy stimulation threshold. Similarly, event sensing thresholds are set to assure the intrinsic events, such as R waves and P waves are detected.
It is desirable to employ pacing energies which are not exorbitantly above the stimulation threshold. The reason for this is that pacemakers are implanted devices and rely solely on battery power. Using pacing energies that are too much above the stimulation threshold would result in early depletion of the battery and hence premature device replacement. Similarly, it is desirable to not render intrinsic event sensing sensitivities too sensitive to avoid sensing noise and other artifacts as the desired intrinsic events.
Operating parameters, such as capture and sensing thresholds are assessed at device implant and periodic follow-up visits with the physician. These processes may be automated. However, very often, atrial capture threshold, ventricular capture threshold, atrial sensing threshold and ventricular sensing threshold evaluation procedures are performed manually by the physicians. During these evaluations, changes in the electrogram signal and/or event markers generated from the electrogram signals are observed and the capture and/or sense thresholds are determined accordingly by the physician. Unfortunately, the changes in the electrogram signals or event markers sometimes can be subtle and the human reaction can be slow to react to them and hence be inaccurate. Some undesirable scenarios may result. These may include, for example, prolonged loss of ventricular capture without ventricular intrinsic support during a ventricular capture threshold test, prolonged loss of atrial capture without atrial intrinsic support during an atrial capture test, incorrect identification of ventricular capture threshold, incorrect identification of atrial capture threshold, missing a true intrinsic event that causes the improper delivery of a pacing pulse, potentially during a vulnerable refractory period, during an atrial or ventricular sensitivity test and incorrect identification of an atrial sensing threshold or a ventricular sensing threshold. Any one of the forgoing would be accompanied by undesirable consequences.
The present invention addresses these an other issues. More particularly, the present invention provides assistance to medical personnel during manual capture threshold and sense threshold evaluations.